188 ANNUAL EEPOKT SMITHSONIAN INSTITUTION^ 1915. 



or molecules, of matter are in close contact, it is a simple matter to 

 deduce the diameter of the atom. This varies slightly for different 

 atoms, but on an average comes out to be about one one-hundred- 

 millionth of a centimeter. It is necessary, however, to be cautious 

 in speaking of the diameter of the atom. The term " diameter of 

 the sphere of action" of the atom is preferable, for it is not at all 

 certain that the actual atomic structure is nearly so extensive as the 

 region through which the atomic forces are appreciable. 



Even before the discovery of the electron the general idea had 

 been suggested that the atom was an electrical structure composed 

 of negatively and positively charged particles held in equilibrium 

 by electrical forces. Such ideas had been proposed and developed 

 by Larmor and Lorentz in order to explain the electrical and optical 

 properties of the atom. The proof that the negative electron was 

 an independent unit of the structure of the atom gave a great im- 

 petus to the formation of more concrete ideas on atomic structure. 

 There was one important difficulty, however, that arose at the outset. 

 While negative electricity had been shown to exist in independent 

 units of very small apparent mass, the corresponding unit of positive 

 electricity was ncA'er found associated with a mass less than the atom 

 of hydrogen. All attempts to show the existence of a positive elec- 

 tron of small mass, which is a counterpart of the negative electron, 

 have resulted in failure, and it seems doubtful whether such a posi- 

 tive electron exists. The role played by positive electricity in the 

 atom was thus a matter of conjecture. In a paper called Aepinus 

 Atomized the late Lord Kelvin considered an atom tc consist of a 

 uniform sphere of positive electrification, throughout which nega- 

 tive electricity was distributed in the form of discrete electrons. 

 In order to make such an atom electrically neutral it is, of course, 

 necessary that the positive charge should be equal and opposite to 

 the charge carried by the electrons. This idea of the structure of the 

 atom was taken up and developed with great mathematical skill by 

 Sir J. J. Thomson. He investigated the constitution of atoms con- 

 taining different numbers of electrons and showed that such model 

 atoms possessed properties very similar to those shown by the actual 

 atoms. The Thomson atom proved for many years very useful in 

 giving a concrete idea of the possible structure of the atom and had 

 the great advantage of being amenable to calculation. 



The rapid advance of science in the last decade has provided us 

 with new and powerful methods of attack on this problem, and has 

 allowed us to distinguish to some extent between various theories of 

 atomic structure. One of these methods depends on the study of the 

 deflection of swiftly moving bodies like alpha and beta particles in 

 their passage through matter. It is found that these rays are always 

 scattered in their passage through matter; i. e., a narrow iJencil of 



